The present invention relates generally to devices and methods for analyzing biological fluids. In particular, it relates to the design and use of improved centrifugal rotors having siphons which allow delivery of a precise volume of liquid to a chamber in the rotor.
Biological tests of blood plasma and other biological fluids frequently require that fluids be quickly divided into predetermined volumes for analysis in a variety of optical tests or assays. It is also frequently desirable to separate potentially interfering cellular components of the material from the other fluid prior to testing. Such measurement and separation steps have previously been typically performed by centrifugation to separate, for instance, blood plasma from the cellular components, followed by manual or automated pipetting of predetermined volumes of the blood plasma into separate test walls. Such procedures are labor intensive and time-consuming. As a result, various automated systems and methods have been proposed for providing multiple aliquots of plasma suitable for testing in a more efficient manner.
A major advance in the analysis of biological fluids has been the use of centrifugal rotors. These rotors are designed to measure volumes of a biological fluid, such as blood, remove cellular components, and mix the fluid with an appropriate diluents for analysis, for example by optical testing. Typically, the rotors provide a plurality of discrete volumes of sample in separate cuvettes in which the sample is optically analyzed.
To ensure accurate and consistent results, such rotors require the delivery of precisely measured volumes of liquid to various chambers in the rotor. This must often be accomplished in circumstances in which the rotor quickly accelerates and decelerates or is otherwise perturbed during operation. This perturbation can often lead to delivery of inaccurately measured volumes. The present invention addresses these and other needs.
U.S. Pat. Nos. 4,894,204 and 5,160,702 disclose siphons for transferring fluids between chambers in a rotor. U.S. Pat. No. 4,244,916 discloses a rotor comprising a plurality of cuvettes positioned radially outward of a central receptacle. Each cuvette is connected to the central receptacle by a duct and comprises a separate air escape orifice. U.S. Pat. No. 4,314,968 relates to rotors having cells positioned on the periphery of the rotor. Each cell includes a peripheral orifice for removing fluid introduced into the cell. U.S. Pat. No. 4,902,479 discloses a multicuvette rotor comprising elongated, radially extending cuvettes. Each elongated cuvette comprises a first chamber for receiving a first constituent and a second chamber for receiving a second constituent. A divider structure between the first and second chambers prevents mixing of the constituents before a predetermined time. Mixing occurs as the rotor is spun at a significant speed. U.S. Pat. No. 4,963,498 discloses devices which rely upon capillaries, chambers, and orifices to pump and mix fluids for optical analysis. U.S. Pat. No. 5,077,013 discloses rotors comprising peripheral cuvettes connected to holding chambers positioned radially inward from the cuvettes.
The present invention provides centrifugal rotors comprising siphons for delivering a premeasured volume of liquid, typically a biological sample such as plasma, between a first and a second chamber in the rotor. The siphons of the invention have an elbow that is radially inward of the radially most inward point of the fluid in the first chamber. As the rotor is spinning the fluid does not flow past the elbow. After the rotor stops, capillary forces xe2x80x9cprimexe2x80x9d the siphon by pulling fluid just around the elbow. When the rotor is restarted, centrifugal force draws the remaining fluid out of the metering chamber into the receiving chamber until the level of the fluid in the metering chamber is at the same radial distance as the outlet of the siphon. The siphons of the invention are designed such that the inlet of the siphon on the first chamber is radially outward or inward of the siphon outlet on the second chamber.
The positioning of the inlets and outlets of the siphons of the invention provide a number of advantages. For example, the inlet of the siphon is always positioned radially outward of the final position of the meniscus of the fluid in the first chamber, after fluid has been transferred to the second chamber. Thus, inaccuracy in measurement associated with different shaped menisci in different fluids is minimized since the meniscus is minimized.
The chambers connected by the siphons of the invention are used to perform any of a number of functions, such as metering liquids, separating solid components from a sample, mixing diluent with the sample, and the like. In the preferred embodiments, the siphons connect a plasma metering chamber to a mixing chamber for mixing the premeasured volume of plasma with diluent.
In addition, the rotors of the invention comprise unmodified inlet channels connecting a distribution ring to cuvettes comprising reagents for optical analysis of a biological sample. The inlet channels are sized such that, as the rotor spins, gas escapes from the cuvette through the inlet channel as the liquid enters the cuvette through the inlet channel. An xe2x80x9cunmodified inlet channelxe2x80x9d as used herein refers to a simple inlet channel, typically having a rectangular cross section, which is not modified (e.g., by altering the cross-sectional shape, surface texture, and the like) to provide a pathway for gas to escape from a cuvette that is not otherwise vented.